1. Field of the Invention
This invention pertains to methods of producing rigid foams and the foams made thereby, particularly polyurethane modified polyisocyanurate foams used for thermal insulation.
2. Prior Art and Other Considerations
Cellular organic plastic foams made with urethane linkages, or made with a combination of both isocyanurate linkages and urethane linkages, are well known in the art. These foams have been made from the catalyzed reaction between polymeric polymethylene polyphenylisocyanate (a.k.a. Polymeric Methylene Di-Isocyanate, or "PMDI") and polyols of various physical and chemical properties. The PMDI has been used either alone, or in a blend with a blowing agent and (optionally) with a capped silicone surfactant. Such a blend utilizing PMDI has traditionally been called the "A-Blend".
In order to form good cell size, good cell distribution, and cell-wall construction, it has sometimes been preferred to add other "plastic foam cell modifiers" to the foam formulations. Often, it has been preferred to add these other agents to the polyol mixture. These foam cell modifiers include, but are not limited to: propylene carbonate, dispersing agents, organic surfactants, predominantly silicone surfactants, nucleating agents, fire retardants, and expansion agents. This blend, including the polyol(s), expansion agent(s), and catalyst(s), has been called the "B-Blend".
As used herein, the term "expansion agents" includes blowing agents and frothing agents. Moreover, as used herein, a blowing agent is a substance which is either produced or becomes a gas subsequent to the first of several chemical reactions. Many blowing agents have boiling points in the range from about 10.degree. C. to about 50.degree. C. On the other hand, CO.sub.2 is considered a blowing agent since, although it has a boiling point outside this range, it is produced by an isocyanate reaction. A frothing agent is a substance which is a liquid under sufficient pressure, then when released from pressurized containment, accordingly produces gas-filled cells in foam prior to the initial chemical reaction. Many frothing agents have boiling points falling in the range of about -52.degree. C. to 10.degree. C.
It has been considered important to keep the viscosity of each mixed blend about equal to the other blend. The rule of thumb has been to keep both the A-Blend and the B-Blend equal in the range of 250 cps to 1500 cps, with the chemical blends at about 60.degree. F. to about 70.degree. F. just prior to mixing. (All viscosities herein are "centipoise" taken on a Brookfield viscometer.)
Prior art insulation thermosetting foams have been primarily "blown" or expanded, by the use of CFC-11, (trichloromonofluoromethane). Some minor use of CFC-12 has also been used, as explained below. Due to environmental considerations, both CFC-11 and CFC-12 have fallen into disfavor. Most commercial foam producers have historically formulated all of their foam formulae around CFC-11 properties. These properties of CFC-11 affecting the foam formulae include the boiling point, the solubility parameters, the number of molecules per unit weight, the latent heat at boiling point, and the rate of membrane permeability of CFC-11.
Likewise, it is important to successful production of foam to have the many complex chemical reactions of the thermosetting polymerization timed with the rate of foam expansion. The dynamics of this problem, along with workable solutions, are taught in U.S. patent application Ser. No. 07/720,735 filed Jun. 25, 1991, and incorporated herein by reference. U.S. patent application Ser. No. 07/720,735 teaches the need for an ethylene linkage in the tertiary amine between two heteroatoms. However, it has since been discovered that another class of tertiary amines have reasonably good hydrogen bonding sites for water, and they will work nearly as well at promoting CO.sub.2 expansion in preference to the urethane reaction. This class is described in some detail below.
When CFC-11 and CFC-12 are ultimately replaced by alternate blowing and/or frothing agents, the prior art techniques will not suffice. The new frothing and blowing agents have detrimental properties that interfere with cell formation. This type of expansion agents include hydrochlorofluorocarbons, or partially hydrogenated chlorofluorocarbons, (referenced by the contraction "HCFCs"); plus the non-chlorine containing fluorocarbons, called hydrofluorocarbons, or "HFCs". As referenced herein, "HFCs" includes HFC-Ethers, as well as plain HFCs. All the physical properties mentioned above (including the boiling point, the solubility parameters, the number of molecules per unit weight, the latent heat at boiling point, and the rate of membrane permeability) differ for HCFCs, HFCs, and HFC-Ethers, as opposed to CFC-11 and CFC-12. As has been shown in U.S. patent application Ser. No. 07/720,735 filed Jun. 25, 1991, these physical properties are detrimental to both heat energy utilization and the timing of the reactions with the rate of expansion.
For example, both HCFC-123 and HCFC-141b have higher boiling points than CFC-11. The CFC-11 boils at 74.9.degree. F. (23.8.degree. C.); while HCFC-123 boils at 82.2.degree. F. (27.9.degree. C.), and HCFC-141b boils at 89.6.degree. F. (32.0.degree. C.). The higher boiling point means the start of the expansion of foam requires more heat energy input than prior art methods. Using prior art methods, these two new HCFCs naturally slow down foam expansion. Slow expansion of the foam allows the chemical reactions to create solidification prior to cell expansion, which causes a high foam density, i.e., low insulating properties.
Another detrimental effect of some new expansion agents is the cooling effect caused by the partial evaporation of low boiling point products. For example, monochlorodifluoromethane, CHClF.sub.2, or HCFC-22, boils at -41.4.degree. F.(-40.8.degree. C.). Some of the product added will evaporate as soon as it is released to atmospheric pressure, and thus will cool the polymer mixture. It has been discovered that the cooling effect of an evaporating frothing agent reduces the exothermic heat from the urethane reaction. To a large degree, the exothermic heat from the urethane reaction is the main heat energy source for the trimerization reaction. It is well known that high levels of heat energy are needed to complete the trimerization reaction which causes the PMDI to form into the isocyanurate linkage. A lack of trimerization causes product failures from the loss of dimensional stability and from excess flammability.
U.S. Pat. No. 4,572,865 teaches the production of polyisocyanurate foams using CFC-12, dichlorodifluoromethane, CCl.sub.2 F.sub.2, which boils at -21.6.degree. F. (-29.8.degree. C.), as a frothing agent. While U.S. Pat. No. 4,572,865 does not specifically mention the cooling effect of using CFC-12, it is well known that this frothing agent does create evaporative cooling in foam production. Other than possibly using high oven temperatures, U.S. Pat. No. 4,572,865 fails to teach any chemical reaction to make up the loss of exothermic heat which is taken away by the evaporative cooling of the frothing agent, CFC-12.
The strong solvent action characteristic of two of the new blowing agents is detrimental if used with methods of the prior art. These two agents are HCFC-123 and HCFC-141b, referred to hereinafter as "2-carbon HCFCs". These two agents are much stronger solvents in both Blends as well as the foam, than were the CFC blowing agents of the prior art. If they are used as the primary expansion agent with prior art polyols, the "solvent effect" of these materials will keep the polymer matrix very soft, and severe shrinking of the foam will occur as soon as the cell gas cools and the vapor pressure is reduced. For example, in U.S. Pat. No. 4,927,863 (Bartlett et al), examples are shown in Table III whereby all prior art foams shrink in excess of 33% when these two HCFCs are used in normal foam blowing amounts. U.S. Pat. No. 4,927,863 teaches a simple method of reducing the amount of the 2-carbon blowing agents utilized, by substituting other expansion agents which have less solvent effect on prior art foam. U.S. Pat. No. 4,927,863 fails to teach any new chemical methods whereby either HCFC-123 or HCFC-141b (or both) may be utilized as the sole expansion agent without the foam shrinking excessively.
As stated above, these new agents are much stronger solvents in both B-Blends and A-Blends than were the CFC blowing agents of the prior art. The increased solubility causes dramatic decreases in Blend viscosities. When the viscosity of the foamable blends gets too low, the resulting mixture of A-Blend (primarily PMDI) with B-Blend (primarily polyol) will form cells with walls which are too weak to hold the cell gas, and they will burst. In any prior art foamable blend system, the viscosity with the 2-carbon blowing agents will get low enough that the resulting mixture will form cells with thin walls and thick intercellular struts. The cell wall becomes thin due to the "drainage" of low viscosity polymer from the wall area to the strut area. This creates a foam which is poor insulation. Very small cell diameters (microcellular), with the cells having closed, thick walls and thin struts, all at the proper density, are desired for good insulation properties. To create good cellular walls in the cellular foam matrix, the viscosity of the final foaming mixture must be high enough to keep the cell from bursting, or to restrain drainage from the cell wall into the cellular strut. This is especially important when frothing agents are utilized which expand instantaneously when released from high pressure containment.
U.S. patent application Ser. No. 07/495,616 filed Mar. 19, 1990 (incorporated herein by reference) teaches that use of a high viscosity polyol with a high number average molecular weight is advantageous to forming strong cell walls when used with HCFCs and HFCs. The polyols described therein all have viscosities over 10,000 cps (Brookfield at 75.degree. F.), and some are over 20,000 cps. It shows that these higher viscosity polyols make blends lower in viscosity than prior art low viscosity polyols make with CFC-11.
It has been known for many years that a smaller, finer cell size and a more rounded shape will produce better thermal insulating properties. For example, U.S. Pat. No. 4,981,879 teaches that perfluorocarbons can be used in minor amounts to produce improved "k-factors". For example, it was reported that the cell diameter of foams using perfluorocarbons (fully fluoronated carbon; i.e., a "FC" has no hydrogen) was about half. Now it has been discovered that utilizing the methods of this invention, any small molecule frothing agent such as HCFC-22 produces microcellularization without the need for a perfluorocarbon. No FC is utilized in this invention.
Thus it is seen that prior art formulations and processing conditions must be significantly changed to utilize HCFCs and HFCs. The new problems presented by the new expansion agents are: 1) Excessively low viscosities and shrinking foam caused by the higher solubility of 2-carbon HCFCs; and, 2) A slow-down of foam expansion due to the higher boiling point of 2-carbon HCFCs; and, 3) A cooling effect from the evaporation of low boiling point frothing agents, reducing needed exothermic heat; and, 4) A cell-rupturing effect due to the rapid expansion of frothing agents.
To compensate for these problems, a novel combination of new polymer materials, new catalyst systems, new blowing agent systems, and new processing conditions are presented in the instant invention.
It is therefore an object of the present invention to provide an improved method for the production of a rigid thermosetting plastic foam insulation.
An advantage of the present invention is the provision of an improved method for the production of a rigid thermosetting plastic foam insulation by using polyols and/or isocyanate prepolymers having ambient temperature viscosities so high they have heretofore been considered unacceptable for use in rigid polyisocyanurate foam production.
Another advantage of the present invention is defeating the detrimental effect of utilizing high percentages of the strong-solvent and high boiling point 2-carbon HCFC compounds.
An advantage of the present invention is the provision of a method of utilizing heretofore unacceptably high viscosity Blends by warming said Blends to reduce viscosities to normal at the mix-head.
An advantage of the present invention is the provision of a method which counteracts the cooling effect of evaporating expansion agents.
Another advantage of the present invention is the provision of a method which provides an increased amount of exothermic heat.
It is another advantage of the present invention to provide a method which conveniently maintains the rate of expansion when utilizing higher boiling point blowing agents which hinder this rate.
It is another advantage of the present invention to provide a method whereby a frothing agent having a lower boiling point and a lower molecular weight than used in prior art foams, as well as blowing agents with higher boiling points, can be used together and still maintain the temperatures needed for the completion of the trimerization reaction as well as maintaining the timing of the speed of foam expansion with the speed of chemical reactions.
It is a further advantage of the present invention to provide an improved cell structure, hence improved k-factors, in rigid plastic foam insulation by utilizing smaller organic molecules in solution than previously used as a nucleating agent in the process.
It is a further object of the present invention to provide a slow rate of blowing agent escaping by diffusion from the cells.
Yet another advantage of the present invention is the provision of a method that compensates for the strong solvent action of some HCFCs and HFCs, and still maintains good cell wall formation.
Yet another advantage of the present invention is the provision of a method that compensates for rapid expansion of a frothing agent by maintaining strong cell walls at the early stages of the chemical reactions and thus producing a high percentage of closed-cells and small cells in the foam.
It is still another object of the present invention to provide an improved structural laminated foam board insulation at a cost lower than would be possible without utilizing a frothing agent.
Another advantage of the present invention is the provision of the use of several common blowing agent materials with low costs, offering the designer multiple choices.
A further advantage of the present invention is the provision of a strong, economical, closed cell foam insulation which is characterized by a high degree of fire resistance, a high initial resistance to thermal conductivity, and a high long-term thermal resistance.